The website got a "soft opening" in Australia with no publicity or advertising.
And within weeks, women in the country have adopted the Ashley Madison agency motto - "life is short, have an affair" - with an enthusiasm founder Noel Biderman says he has not seen before.
"We've never had more women than men, it's always been about two to one, male to female (membership)," the Sydney Morning Herald quoted Biderman as saying.
"Australia has this unique factor, which could be an early adopter situation, it's about 36 per cent female, and that's about six per cent more than in any other city we've ever encountered on the female side," he added.
Biderman- known in the US as the "King Pin of Infidelity"- officially launched the Australian version of his online dating service for married or attached men and women.
However, he defended his controversial venture, saying he is surely doing something right to boast of a membership of 5.5 million.
Biderman said 40,000 Australians so far had joined the agency, which he started in Canada in 2001, fusing the two most popular children's names at the time to come up with AshleyMadison.com.
And he said that the membership is expected to grow to more than one million once a provocative television, billboard and online advertising campaign kicks off.
Biderman said men and women joined for different reasons.
"I think ultimately for men it's about sex, this is about an absence of sex in their lives or sex being too 'vanilla' and they're looking for different flavours. I think with women it's a bit more complicated, to be honest with you," he said.
Some women were looking for the "lifestyle benefits" that might accompany the mistress status, while others wanted to continue being objects of desire, said Biderman.
"They're not being brought flowers anymore and they're not being paid attention to so they're looking to rekindle that feeling," he said.